The chest x-ray is the most commonly performed diagnostic x-ray examination. In chest x-ray imaging, the heart, lungs, airway, blood vessels and the bones of the spine and chest are imaged in a painless medical test that helps in the diagnosis of medical conditions.
The chest x-ray is typically the first imaging test used to help diagnose causes of symptoms such as shortness of breath, fever, a bad or persistent cough, chest pain or injury. Its application helps in diagnosing and monitoring treatment for medical conditions such as pneumonia, lung cancer, emphysema, heart failure and other heart problems. It may be used to find fractures in ribs as well.
Pneumonia shows up on radiographs as patches and irregular whiter areas due to higher x-ray absorption from fluid in the lungs. If the bronchi, which are usually not visible, can be seen, a diagnosis of bronchial pneumonia may be made. Shifts or shadows in the region corresponding to the hila (the central mediastinal surfaces of the lungs where the lung roots enter) may indicate emphysema or a pulmonary abscess. Apparent widening of the spaces between ribs suggests emphysema. Chest x-ray examination may aid in the diagnosis of other pulmonary diseases.
Lung cancer usually shows up as some sort of abnormality on the chest radiograph. Hilar masses (enlargements at that part of the lungs where vessels and nerves enter) are one of the more common symptoms of lung cancer that may be shown in chest x-rays. Another common symptom is the indication of abnormal masses and fluid buildup on the outside surface of the lungs or surrounding areas. Interstitial lung disease, which is a term relating to a large category of disorders, many of which are related to exposure of substances such as asbestos fibers, for example, may be detected on a chest x-ray as fiber like deposits, often in the lower portions of the lungs.
One reason for conducting chest x-rays is to look for nodules or lesions, which are typically spherical regions of densification of the lung tissue that are often malignant and are indicative of cancer. The lungs themselves are situated within the rib cage. Because bones absorb x-rays to a larger extent than soft tissues, rib shadows appear as white stripes across the x-ray radiograph. Since the ribs curl around the body, the rib shadows as they appear on the x-ray radiograph, appear to cross each other. The cross-over points and the regions between the cross-over points appear as a pattern of almost parallelogram-like shapes. This pattern may disguise the shadows cast by nodules which may be aligned with the crossover points, with the spaces therebetween, with a single rib between the cross-over points and may overlap two or three of these regions. Thus the shadows cast by the ribs may disguise and conceal nodules.
In addition to the rib cage, the blood vessels within the lung have higher density than the air filled spongy lung tissue and absorb more x-rays, thereby causing them to cast a shadow, i.e. appear lighter on the radiograph. A blood vessel aligned head on to the direction of photography may cast a circular shadow and be mistaken for a nodule.
Absolute absorption varies with the period of exposure, the strength of the x-ray source and the size and shape of the patient. Details are resolved by identifying contrast discontinuities between regions, and computer aided diagnostic (CAD) programs have been developed to aid the process.
Whilst being x-rayed, patients are supposed to keep still and hold their breadth, thereby keeping lungs inflated. However, in practice patients may inadvertently move or inhale or exhale during the exposure to x-rays, blurring the resultant radiography images somewhat. Furthermore, since x-ray radiation may itself cause malignancies, radiologists are wary of repeatedly exposing patients to x-rays, particularly with pregnant women, the young, the elderly and/or the ill. Sometimes, it may be impossible to repeat tests once a problem is determined, as the patient may no longer be available.
Lung cancer may manifest itself as one or more malignant nodules which are typically near-spherical and may show up as near-circular whiter regions on the lung tissue. The size of a nodule may be smaller, similar or larger than the rib cross-over, and thus isolation and identification of nodules is not easy.
It is, of course, imperative to detect all resolvable nodules. To avoid unnecessary anxiety and to reduce costs of unnecessary biopsies; it is desirable to minimize false positives as well.
The skilled radiographer may manually pick out nodules or lesions, but to speed up the diagnostic procedure, Computer Aided Diagnostic (CAD) techniques are increasingly used.
One approach used is segmentation, where the borders of features such as the lung lobe itself and ribs are identified. This is not always possible and indeed, is not always necessary.
What is required is to identify candidate regions of interest and to then decide if they are nodules or false positives. Once a nodule is identified, it is then necessary to determine if it is benign or malignant. This may require a biopsy, but a determination may sometimes be made by monitoring the nodule over time.
‘False negatives’ where actual nodules or tumors are not identified as such or discarded as ‘noise’ or some artifact of the system are unacceptable since late diagnosis may lead to complications and such mistakes may be fatal. The occurrence of ‘false positives’ where some other feature such as a rib crossing shadow is mistakenly identified as being attributed to a tumor, is less problematic but is still costly and may cause unnecessary anxiety to the patient so should also be minimized. Despite the importance, in practice the elimination of false positives whilst avoiding false negatives is difficult since tumors may have similar dimensions to the width of ribs or the diameter of blood vessels within the lung, and x-ray images typically have poor contrast. When scanning the lung region of x-ray images to look for regions of varying contrast or edges, a large number of candidate regions turn out to be rib crossings and the like.
There is a need to identify and eliminate rib shadows from chest x-rays and mammography images. Explicit rib segmentation is one approached that has been tried in the past and has met with some success however there is often an imprecision in defining the ribs and it will be appreciated that segmentation of the ribs as a precursor to suppression thereof is a computer intensive procedure that expends large amounts of processing power on defining and resolving features to be discarded which is a somewhat inefficient approach. Additionally, rib suppression based on segmentation which makes use of the varying contrast (grey levels) in the vicinity of the ribs, often results in distortions of the lung tissue which may introduce artifacts, i.e. noise from the processing. Such distortions may conceal elements of interest as well. Consequently, other methods of rib suppression are required and embodiments of the present invention address this need.